Photolithography is one of the most critical steps in IC manufacturing process. The quantity of photomask and lithography needed in the process indicates the integration complication of the IC. The improvement of lithography determines whether the IC can have a smaller feature dimension than previous generation. Exposure of photo-resist is one of the critical steps in lithography process due to its accuracy requirement. As mentioned above, the IC manufacturing process involves typically more than ten photomasks of different patterns, the alignment between the photomasks critically determines the quality of patterns transferred to the target layer and the final performance of the IC.
As well known in the arts, the step and repeat projection exposure step is to scale down for 4 to 10 times the patterns on the photomask on surface of substrate for each step, and multiple steps are repeated to complete the exposure for the entire substrate of wafer, as indicated in FIG. 1. The apparatus used for executing the exposure step is the so-called Stepper, which typically employing charge-coupled diode (CCD) camera for alignment purpose. As illustrated in FIG. 1, light source 11 typically may be the Ultra-Violet light emitted by mercury arc lamp, or other light source facilitating the same purpose. The photomask 12 has device patterns thereon, and additional overlay mark 13 positioned at the edge of the photomask 12, the alignment must be assured before each exposure is performed. As indicated, through the lens 14, the device patterns on the photomask 12 and the overlay mark 13 are transferred to the surface of the wafer 15. Compared to the actual size, the pattern on the photomask 13 is magnified, and the pattern is shrunk to actual size by lens 14 and transferred on one location of the wafer during exposure step. Since the lithography process is well known in the arts, the details of the process are not further described herein.
The well known overlay marks, such as Box-in-Box (BiB) and Advanced Imaging Metrology (AIM) are generally used for the overlaying alignment between photomasks. As indicated in FIG. 2A, a BiB overlay mark 20 according to the prior arts is provided on the photomask as having a first rectangular region 21, a second rectangular region 22, a third rectangular region 23 and a fourth rectangular region 24. The longer side of the first rectangular region 21 and the longer side of the third rectangular region 23 are parallel to each other, the longer side of the second rectangular region 22 and the longer side of the fourth rectangular region 24 are parallel to each other. The longer side of the first rectangular region 21 or the third rectangular region 23 is perpendicular to the longer side of the second rectangular region 22 or the fourth rectangular region 24. Namely, along the horizontal and vertical directions, there are two parallel and symmetrical rectangular regions respectively. Alternatively, these four regions may be independently an open region or may be connected to each other by joining tail of one region to head of the connecting region.
FIG. 2B illustrates a mark pattern 30 previously formed on the substrate after the previous process is completed. The mark pattern 30 includes a first aligned rectangular region 31a, a second aligned rectangular region 32a, a third aligned rectangular region 33a and a fourth aligned rectangular region 34a. 
The alignment configuration 40 is illustrated in FIG. 2C. As mentioned above, the pattern in FIG. 2A is transferred on the photo-resist to form a first rectangular region 31b, a second rectangular region 32b, a third rectangular region 33b and a fourth rectangular region 34b. By making measurement of orientation and/or gap between the first aligned rectangular region 31a, the second aligned rectangular region 32a, the third aligned rectangular region 33a and the fourth aligned rectangular region 34a and the first rectangular region 31b, the second rectangular region 32b, the third rectangular region 33b and the fourth rectangular region 34b respectively, the alignment step is performed. When the measured orientation and/or gap meet the predetermined criterion, the patternization is successful and process continues. However, when criterion is not met, the failure photo-resist at this stage must be removed and the lithography process is repeated again until the criterion is met.
The AIM overlay mark is designed using the optical metrology and is more dense than BiB overlay mark for reducing possible inaccuracy during manufacturing process. In addition, the BiB overlay mark on the layer of wafer may be eroded after processes including etching, CMP or ion implant such that alignment procedure can not be performed accurately. In contrast, the AIM overlay mark has lesser open area lowering possibility of erosion during processes and, therefore, enhancing alignment accuracy between photomasks.
As indicated in FIG. 3A, an AIM overlay mark 50 according to the prior arts is provided on the photomask as having a first rectangular region 51, a second rectangular region 52, a third rectangular region 53 and a fourth rectangular region 54. The longer side of the first rectangular region 51 and the longer side of the third rectangular region 53 are parallel to each other, the longer side of the second rectangular region 52 and the longer side of the fourth rectangular region 54 are parallel to each other. The longer side of the first rectangular region 51 or the third rectangular region 53 is perpendicular to the longer side of the second rectangular region 52 or the fourth rectangular region 54. Namely, along the horizontal and vertical directions, there are two parallel and symmetrical rectangular regions respectively.
FIG. 3B illustrates a mark pattern 60 previously formed on the substrate after the previous process is completed. The mark pattern 60 includes a first aligned rectangular region 61a, a second aligned rectangular region 62a, a third aligned rectangular region 63a and a fourth aligned rectangular region 64a. 
The alignment configuration 70 is illustrated in FIG. 3C. As mentioned above, the pattern in FIG. 3A is transferred on the photo-resist to form a first rectangular region 61b, a second rectangular region 62b, a third rectangular region 63b and a fourth rectangular region 64b. By making measurement of orientation and/or gap between the first aligned rectangular region 61a, the second aligned rectangular region 62a, the third aligned rectangular region 63a and the fourth aligned rectangular region 64a and the first rectangular region 61b, the second rectangular region 62b, the third rectangular region 63b and the fourth rectangular region 64b respectively, the alignment step is performed. When the measured orientation and/or gap meet the predetermined criterion, the patternization is successful and process continues. However, when criterion is not met, the failure photo-resist at this stage must be removed and the lithography process is repeated again until the criterion is met.
Due to continuing demand for further down-scaling of IC process, further accuracy of photomasks alignment becomes an important issue which is to be resolved by any improved overlay mark configuration such as that disclosed by this application.